This invention relates to a stepless belt transmission mainly for automotive vehicles.
A stepless belt transmission is equipped with a transmission mechanism comprising input and output pulleys of variable effective diameter each having a movable flange and stationary flange, a transmission belt linking the input and output pulleys, and a hydraulic actuator for varying the effective diameter of the input or output pulley to control the transmission ratio. The stepless belt transmission also includes a control unit equipped with a transmission ratio control mechanism which effects shifting by supervising the supply and discharge of the working fluid to and from the actuator in dependence upon such vehicle running conditions as vehicle speed, throttle opening, selected speed range and output shaft torque. Transmission ratio control mechanisms known in the art include an electromagnetic solenoid valve-spool valve combination, as disclosed in the specification of Japanese Patent Application Laid-Open No. 58-57552, and a configuration in which a spool valve is coupled to a movable flange by linkage, as set forth in the specification of Japanese Patent Application Laid-Open No. 57-161345.
While the arrangement relying upon the combination of solenoid and spool valves is simple in structure and of compact size, there is a tendency for control to become overly complicated when it is attempted to perform a complex shifting operation. In the arrangement having the spool valve linked to the movable flange, a space limitation is imposed in terms of the structure of the linkage. Another problem is that the coupled portions tend to rattle.